1. Field of the Invention
The invention relates generally to the field of linear actuators. More specifically, the invention relates to linear actuators using magnetostrictive elements to generate linear motion.
2. Background Art
Linear actuators have wide application in devices used in connection with wellbores drilled through the Earth's subsurface. For example, such actuators are known in the art be used to operate subsurface safety valves or other valves. Such actuators are also known in the art to be used to open and close back up arms or pads on well logging devices, or to actuate steering devices in certain drilling tools such as rotary steerable directional drilling systems.
Irrespective of the particular use, linear actuators used in connection with wellbore devices are most commonly of two types. One type includes a motor that drives a screw or worm gear. The screw or worm gear is coupled to a ball nut. Rotation of the screw is translated into linear motion of the ball nut. See, e.g., U.S. Pat. No. 6,898,994 issued to Walton.
The other type of actuator in widespread use is hydraulic. Typically, a motor drives an hydraulic pump, and pressure from the pump (which may be stored in an accumulator or similar reservoir) is selectively applied to one side or the other of a piston disposed in a cylinder. The force of the pressurized hydraulic fluid acting on the piston moves the piston along the cylinder. See, e.g., U.S. Pat. No. 5,673,763 issued to Thorp and presently owned by the assignee of the present invention.
Electric linear actuators are known in the art. See, e.g., U.S. Pat. No. 6,100,609 issued to Weber. Many electric linear actuators operate on a principle similar to the motor/ball screw combination referred to above. Typically such combination of motor and reduction gear is necessary for the actuator to produce the force required to operate the wellbore device. Electric linear motors known in the art are typically unable to produce such force absent a reduction gear.
More recently, magnetostrictive elements have been used to produce a linear actuator. See, for example, Won-Jong Kim et al., Extended-Range Linear Magnetostrictive Motor with Double-Sided Three-Phase Stator”, IEEE Transactions on Industry Applications vol. 38, no. 3 (May/June 2002). The actuator described in the foregoing publication uses, for a power producing element, a magnetostrictive material such as one sold under the trademark ETREMA TERFENOL-D, which is a registered trademark of Edge Technologies, Inc., Ames, Iowa. The magnetostrictive element is in the form of a rectangular slab placed between two tight-fitting armatures. The armatures are subjected to a magnetic field generated by multiphase alternating current, such that the magnetic field “moves” in a manner similar to that of an electric induction motor. The magnetic field alternatingly causes magnetostriction of part of the magnetostrictive element and its consequent elongation normal to the magnetostriction, while other parts of the magnetostrictive element remain tightly held within the armatures. The friction between the stationary armatures and the uncompressed part of the magnetostrictive element provides the reaction force required to move the elongating part of magnetostrictive element against a load, causing the load to move. By “moving” the magnetic field along the armatures, the magnetostrictive element undergoes peristaltic or “inchworm” like motion, thus moving the load.
The foregoing linear actuator has been difficult to adapt to wellbore devices because of its shape.
Another type of linear actuator using a magnetostrictive material element is disclosed in Bryon D. J. Snodgrass, Precision moves with magnetostriction, MachineDesign.com (Nov. 18, 2004). The foregoing actuator does not have any mechanism to compensate for thermal expansion or wear of moving elements.
There continues to be a need to improved linear actuators for use in connection with wellbore devices.